1. Field of the Invention
The present invention relates to a photographing object lens enabling photographing at a close distance, particularly those employing the close distance correction system which enables to photography from infinity to equi-magnification.
2. Related Background Art
Various lens systems have been proposed which enable taking photographs at a distance from infinity to extremely close distance. Among such lens systems are (a) those which employ 3 group composition comprising positive 1st group, positive 2nd group and negative third group and focus the system by zooming out all three groups independently to the object side, as disclosed in Japanese Patent Application Laid-Open No. 59-228220, U.S. Pat. No. 4,786,153, No. 4,464,023; (b) (b) those which focus by zooming out the positive 1st group and the positive 2nd group while fixing the negative 3rd group, as disclosed in (1) U.S. Pat. No. 4,392,724, (2) Japanese Patent Application Laid-Open No. 58-34418, and (3) Japanese Patent Application Laid-Open No. 59-152414; and (c) those which focus by zooming out the positive 1st group and positive 2nd group as one set to the object side while fixing the 3rd negative group, as disclosed in Japanese Patent Application Laid-Open No. 63-179308.
These known lens systems are called micro-lens or macro-lens employing floating systems (hereinafter collectively called "micro-lens") and they enable taking photographs at a distance from infinity to the close distance where magnification is equi-magnification to about 1/2 scale. The aforesaid lens system (a) wherein all three lens groups move has an advantage in that it provides a large freedom for correction of varying aberration at the time of close distance photograph, as the number of movable lens groups is large, but on the other hand it poses a problem in that the focussing mechanism is complicated and the common difference of eccentricity of lens groups is rigid.
While the aforesaid lens system (c) has a simple focussing mechanism, the freedom of correction of aberration is too small and optical performance is not sufficient for focussing for a wide distance range as from infinity to equi-magnification.
In the aforesaid lens system (b), the system (1) disclosed in U.S. Pat. No. 4,392,724 enables to photography up to equi-magnification but the 3rd group has an extremely complicated lens composition. In system (2) disclosed in Japanese Patent Application Laid-Open No. 58-34418, the 3rd group has a relatively simple composition but since the magnification shared by the 3rd group is relatively high, the closest distance where photographing can still be done at a favorable state of aberration correction is not more than about 1/2 magnification. The system (3) disclosed in Japanese Patent Application Laid-Open No. 59-152414, has a drawback in that the magnification effected by the 3rd group is too low and consequently the maximum amount of zooming out of the 1st group must be made large and the lens diameter of the fixed 3rd group must be made large in order to secure peripheral light quantity in the case of extremely close distance photographing.